Among processes for forming and modifying organosiloxanes, hydrosilylation catalyzed by transition metals is of particular importance since hydrosilylations make a wide variety of SiC linkages possible. However, regardless of the wide application opportunities for this reaction, industrial implementation of prior art hydrosilylations is frequently accompanied by considerable difficulties. Central to these problems is the catalyst activity which can change over time and is subject to many interfering influences. Since hydrosilylation reactions liberate energy (i.e., the reactions are exothermic), a fluctuating catalyst activity in batch processes not infrequently leads to critical operational states because reactants can accumulate in between and can thus build up a hazard potential. The batch process represents the customary state of the art for industrial hydrosilylations, not least because experience has shown that a preactivation phase to form the catalytically active species from the inactive precursor material is absolutely necessary. The fluctuating reaction behaviors of industrial hydrosilylation reactions require the presence of many qualified employees.
One particular hazard potential results from the material systems in hydrosilylation processes, which systems have a high hydride hydrogen density. Apart from monomeric silanes, specific mention is made of the derivatives of poly(methylhydrogen)siloxane.
Along with the hazard potential, the critical systems also give undesired by-products, i.e., the selectivity of the intended reaction suffers.
The known chemical processes for preparing organofunctionalized poly(methylhydrogen)siloxanes therefore make conspicuous efforts to minimize the steady-state concentration of active SiH groups in the SiC linkage step.
For example, WO 98/05700 describes a (semi)continuous process for preparing multifunctional polyorganosiloxanes containing Si-alkyl and Si-alkoxy groups in a multistage reaction apparatus comprising a combination of a dehydrocondensation reactor with a hydrosilylation reactor. The dehydrocondensation reactor is supplied with an SiH-containing polyorganosiloxane and a deficiency, based on the polyorganosiloxane, of an alkyl or thioalkyl in the presence of a platinum catalyst, forming, with liberation of H.sub.2, a mixed alkoxyhydrogensiloxane or thioalkoxyhydrogensiloxane which is then immediately subjected to alkylation by an olefin to form an SiC bond in a downstream hydrosilylation reactor. Attention is paid in WO 98/05700 (see, Page 2, line 29-Page 3, line 3) to the hazard potential involved in handling SiH-containing compounds, even compounds having partly alkoxy- or thioalkoxy-functionalized chains. For this reason, the actual hydrosilylation is therefore deliberately carried out only after the partially deactivating reaction (dehydrocondensation); operation of this process brings with it the technical problem of ensuring complete removal of alcohol/thiol and hydrogen gas prior to the hydrosilylation. In terms of the apparatus, a complicated and costly solution is therefore necessary.
Continuous hydrosilylation processes which are already known per se also seek to lower the in-situ concentration of active SiH groups. DE 196 32 157 A discloses a process for the continuous preparation of organosilicon compounds of the 3-halopropyl-organosilane type having the general structure R.sub.b H.sub.3-a-b X.sub.a SiCH.sub.2 CH.sub.2 CH.sub.2 Y by reaction of an allyl halide with an excess of a silane having at least one H atom. Propyl-organosilanes which cannot be utilized are formed as undesired by-products. The essential feature of this prior art process is that by-product formation is suppressed by setting a partial conversion of the starting materials of 10-80%, based on the component present in a deficiency.
A further process for continuously carrying out hydrosilylation reactions is disclosed in DE 196 19 138 A. This document describes a process for preparing vinylated organosilicon compounds in which an organosilicon compound containing at least one SiH group is reacted with an excess of acetylene in a largely inert liquid phase in the presence of a catalyst. To ensure intensive mixing of the reaction matrix, a jet loop reactor is employed.
Continuous gas-phase processes in a flow reactor which have been previously described include: the molecular addition of acetylene onto methyldichlorosilane over a Wilkinson catalyst immobilized on chrysotile asbestos (Appl. Organomet. Chem. (1987), 1 (5), 459-63); and the hydrosilylation of acetylene using trichlorosilane over Rh- and Ru-phosphine complexes fixed to SiO.sub.2, which proceeds with limited selectivity (Appl. Organomet. Chem. (1987), 1 (3), 267-73).
The prior art thus documents three methods of diluting active SiH groups:
(i) the partial conversion procedure; PA0 (ii) the use of an inert liquid phase; and PA0 (iii) the reaction in a gas space.